§1.1 Field of the Invention
The present invention concerns biological and/or chemical sensing. In particular, the present invention concerns floating gate (e.g., silicon or organic) transistors, such as Ion Sensitive Field Effect Transistors (ISFETs) for example, for chemical and/or biological sensing.
§1.2 Background Information
The key element of a conventional silicon-based ISFET is a gate capacitor formed between the gate and substrate. The electric field within the gate (or oxide) capacitor is determined by the difference in work function of the plate materials forming the gate and silicon substrate. The electric field within the gate oxide determines the amount of surface charge near the silicon and oxide interface and sets the conductivity of the field effect transistor (FET) (between source and drain).
If the gate of an ISFET is formed by a material which is sensitive to selective gases or analytes, the electric field within gate oxide will be determined by the electrochemical properties of the combined (gate and gas or analyte) system. This mechanism has been exploited to design various types of silicon based ISFETs. (See, e.g., C. G. Jakobson, U. Dinnar, M. Feinsod, and Y. Nemirovsky, “Ion-Sensitive Field-Effect Transistors in Standard CMOS Fabrication by Post Processing,” IEEE Sensors Journal, (2002); and J. Janata, “Electrochemical Microsensors,” Proceedings of the IEEE, Vol. 91(6), pages 864-869 (2003), both incorporated herein by reference.)
FIG. 1 depicts a conventional ion sensitive field effect transistor (ISFET) submerged in gas or analyte. Specifically, silicon based ISFET 120 comprising a gate 130, a source 124, a drain 126, a silicon oxide layer 128, and a silicon substrate 122 is exposed to a sample (gas or analyte) in a container 110 with a reference electrode 140 so as to detect the presence and/or amount or concentration of a target in the sample.
Although conventional Si-based ISFETs can sense various types of targets, there are some limitations that prevent their widespread deployment. For example, in a conventional scheme for ISFET-based sensing, such as that shown in FIG. 1, the gate 130 voltage is set by the electro-chemical properties of the sensing material and the bias voltage of a reference electrode 140. Due to this reference electrode-based biasing scheme, these ISFETs are suitable as pH sensors, but they have limited use as gas sensors.
To overcome some of these limitations, the electrical properties of the conductive channel of ISFETs can be modulated directly, without the intervention of gate electrode, as shown in FIG. 2. FIG. 2 depicts a conducting polymer-based ISFET 210 exposed to gas or analyte. Specifically, an organic-based ISFET 210 comprises a gate 220, a source 214, a drain 216, an oxide layer 218, and a conducting polymer substrate 212 (which includes conductive channel from source 214 to drain 216). This ISFET 210 may be exposed to a sample (gas or analyte) so as to detect the presence and/or amount or concentration of a target in the sample. Although the use of conducting polymer-based ISFETs simplifies the sensing measurements, only those ISFETs having a conducting polymer as the channel/substrate material have shown promising results. Unfortunately, organic FETs generally have a much lower drive current compared to bulk silicon devices. Consequently, a comparatively large (e.g., high device width/length ratio) device is required for acceptable drive current to suppress the affect of background noise and to minimize the need for significant amplification of the detected signal. It is also difficult to integrate on-chip bias and peripheral circuits with organic FETs, and they are not as easily miniaturizable as silicon FETs.
In view of the foregoing disadvantages of known transistor-based sensors, it would be useful to provide improved sensors which overcome one or more of such disadvantages.